Data storage media are commonly used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks or cards, and the like. In magnetic media, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the medium is often organized along “data tracks,” and transducer heads are positioned relative to the data tracks to write data to the tracks or read data from the tracks. A typical magnetic storage medium, such as magnetic tape, typically includes several data tracks. Optical media, holographic media and other media formats may also make use of data tracks.
As the number of data tracks on a medium increases, the data storage capacity of the medium likewise increases. However, as the number of data tracks increases, the tracks usually become narrower and more crowded on the medium surface. Accordingly, an increase in the number of data tracks can make positioning of the transducer head relative to a desired data track more challenging. In particular, for proper data storage and recovery, the transducer head must locate each data track, and follow the path of the data track accurately along the media surface. In order to facilitate precise positioning of the transducer head relative to the data tracks, servo techniques have been developed.
Servo marks refer to signals, patterns or other recorded markings on the medium that are used for tracking purposes. In other words, servo marks are recorded on the medium to provide reference points relative to the data tracks. A servo controller may interpret detected servo marks and generate position error signals. The position error signals are used to adjust the lateral distance of the transducer head relative to the data tracks so that the transducer head is properly positioned along the data tracks for effective reading and/or writing of the data.
With some data storage media, such as magnetic tape, the servo marks are stored in specialized tracks on the medium, called “servo tracks.” Servo tracks serve as references for the servo controller. Conventional servo tracks typically hold no data except for information that is useful to the servo controller to identify positioning of a transducer head relative to the surface of the medium.
The servo marks recorded in the servo tracks may be sensed by one or more servo heads. For example, servo heads may be dedicated heads that read only servo marks in the servo tracks. Once a particular servo track is located by the servo head, a data track can be located on the medium according to the data track's displacement from the servo track. The servo controller receives detected servo signals from the servo heads, and generates position error signals, which are used to adjust positioning of a transducer head relative to the data tracks.
Time-based servo techniques refer to servo techniques that make use of time variables. Time-based servo techniques are particularly effective for magnetic tape, which typically feeds past transducer heads at a constant velocity. For example, N-shaped servo markings, servo markings such as “<<< >>>” or “//// \\\\,” or the like, have been developed for time-based servo techniques. Such markings are typically formed in a servo track of the magnetic tape.
When time-based servo techniques are used, the time offset between detection of two or more servo marks can be translated into a position error signal, which defines a lateral distance of the transducer head relative to a data track. For example, given a constant velocity of magnetic tape formed with marking “/ \”, the time between detection of “/” and “\” becomes larger when the servo head is positioned towards the bottom of marking “/ \” and smaller if the servo head positioned towards the top of marking “/ \”. Given a constant velocity of magnetic tape, a defined time period between detected servo signals may correspond to a center of marking “/ \”. By locating the center of marking “/ \”, a known distance between the center of the servo track and the data tracks can be identified.